The present invention relates to the digital transmission of information and more particularly to the phasing of digital trains with a view to satisfying a switching order, said trains containing identical data signals and clock signals of the same frequency.
Modern digital transmission systems require processing, maintenance and safety systems of increasingly higher performance, all the more so as the rates increase (2 Mbit/s, 8 Mbit/s, 34 Mbit/s, 2.times.34 Mbit/s, 140 Mbit/s). Such requirements are also met with in the transmission of information both by radio beams and by electrical cables or optical fibres.
The device according to the invention is applied to any digital transmission system conducting digital information via two different paths, the choice of the path being a function of determined criteria particular to the system or being controlled from the outside.
In particular, in the digital transmission systems employing radio beans, a backup channel is added to the n channels in operation, which backup channel is capable of replacing an unavailable channel over a whole switching section. The usual reasons for such switching orders are fadings on a service channel, maintenance or defects.
A digital train switching device has already been described in Applicant's French Patent Application No. 79 18 479. It evokes the necessity that switching does not introduce any additional transition in the clock signal. In fact, such a switching must be effected very rapidly after a demand issuing from the reception logic further for example to the result furnished by a quality appreciator, the order of switching being transmitted due to the order channels. This switching must also be effected without loss nor addition of information. This condition is imperative as such a defect would cause losses of synchronisation of the downstream equipment.
Millions of bits could be lost in this way. Moreover, it is desirable that this switching does not introduce any erroneous information, in particular in the case of handling the backup channel for servicing.
Pat. No. 79 18 479 makes it possible to smooth the clock phase jump at the moment of switching, but only if this jump is not too considerable.
However, it is very difficult to foresee the phase difference of the clock signals and the data of each channel, a backup channel having to be operational virtually instantaneously and equally for n channels.
In fact, the various digital trains present shifts which may attain several clock periods due to the differences in transmission medium, ageing of the equipment, variations in temperature, etc.
It is therefore necessary to prepare the trains to be switched in order to adapt them in phase before subjecting them to the switching of the Patent mentioned above. Furthermore, the phase adaptation of the trains must be compatible with the gauges admissible by the downstream equipment, for example the interfaces.
French Patent Application No. 2 462 065 in particular teaches effecting switching only after the phasing of the clock signals and the synchronism of the binary rates of the signals. A coincidence test is also effected, but only one shift order is sent each time a predetermined number of non-coincidences are observed, until there is effectively coincidence.
The present invention concerns more particularly the tests of preparation for making a demand for phasing of the digital trains.
It is another object of the present invention, further to the tests and to a demand for phasing, to effect actual phasing satisfactorily, whatever the contents of the digital trains.
The object of the invention is, on the one hand, to control the phase changes, and, on the other hand, to carry them out without risk of exceeding the jitter admissible for the downstream equipment.